Fluid mesh disappears when I save
Inside the fluid simulation settings, go into the Cache menu. Inside of it there should be a file path. This will lead to the location where your fluid data is stored in.
Navigate to the folder where the Cache says the data is. Inside that folder, look for more folders titled with names like Data, Mesh, Config as those are the folder names that Blender uses to store fluid sim data. If you are able to find these folders and they have files inside, then you have a problem because Blender is not reading them. Double-check the file paths.
If they don't exist, then you have a problem because the fluid simulation files are gone. This is usually because if you do not specify a cache folder, then Blender finds a temporary space for it. On my computer, this temporary space is always cleared when it shuts down or restarts. This means you have to rebake everything again.
To avoid this problem in the future, always specify a cache folder in the cache menu before you bake anything.